Human monocarboxylate transporters (hMCTs) mediate the transport of monocarboxylates across plasma membranes. One such transporter, hMCT9, has been shown to be related to serum uric acid levels and the risk of renal overload gout. However, the functional characteristics of hMCT9 remain unknown. The aim of this study was to investigate the expression and localization of hMCT9 using a Xenopus laevis oocyte heterologous expression system and characterize its transport properties. Kinetic analysis of hMCT9-mediated creatine uptake revealed that uptake consisted of two components, with apparent K-m values of 237 mm (low-affinity) and 23.7 mm (high-affinity), respectively. The transport activity of hMCT9 was dependent on the extracellular pH and activity sharply increased with increasing pH. Under Na+-free conditions, hMCT9-mediated creatine uptake was reduced by one-half, indicating that hMCT9 is a Natsensitive transporter. Moreover, carbonyl cyanide 3-chlorophenylhydrazone (a protonophore) inhibited hMCT9 activity, whereas valinomycin (a K+-ionophore) did not inhibit the transporter. These results suggest that hMCT9 is susceptible to changes in H+ gradients. A cis-inhibition assay of hMCT9-and hMCT12-mediated creatine transport revealed that cyclocreatine, creatine, guanidineacetate, and 3-guanidinopropionate are recognized by the transporter, and 4-guanidinobutyrate and guanidinoethyl sulfonate selectively inhibited hMCT9 activity. These findings demonstrate that hMCT9 is an extracellular pH- and Na+-sensitive creatine transporter. (C) 2019 Published by Elsevier Inc.